How do I make my obj import operator preset the default option?
TL;DR: Editing this file works:
C:\Program Files\Blender Foundation\Blender 2.83\2.83\scripts\addons\io_scene_obj\__init__.py
Long version: How to edit the file:
This file is a text description of the UI elements a user sees when selecting file &gt; import &gt; wavefront (.obj).
If you look through the file, you will see class ImportOBJ and within that class, several blocks of text called &quot;BoolProperty&quot; or &quot;StringProperty&quot; or &quot;EnumProperty&quot;.
Each of these blocks of text corresponds to an element you see in the &quot;Blender File View&quot; window operator presets panel:

In my case, I was looking to alter the default value for &quot;split/keep vert order&quot; (because changing vert order on import is very unwise, as it's destructive), which was &quot;ON&quot; (split) in my distro (2.83.10).  I changed it to &quot;OFF&quot; (Keep Vert Order):
    split_mode: EnumProperty(
        name=&quot;Split&quot;,
        items=(('ON', &quot;Split&quot;, &quot;Split geometry, omits unused verts&quot;),
               ('OFF', &quot;Keep Vert Order&quot;, &quot;Keep vertex order from file&quot;),
               ),
        default='OFF',
        )

You can use this technique to change the way the UI appears in many ways.  This is just one small example, specific to this question.